The quality of modulated optical signals transmitted in long-distance fiberoptic communication systems is frequently characterized by optical signal-to-noise ratio (OSNR), which defines a ratio of the total optical power of the digital information signal to optical noise added to the signal by optical amplifiers. In communication systems with only a few widely-spaced wavelength-multiplexed signals, OSNR may be readily determined by spectral analysis of a transmitted noisy signal and the optical noise floor on either side of the signal spectrum. By way of example, International Electrotechnical Commission Standards Document “Digital systems—Optical signal-to-noise ration measurements for dense wavelength-division multiplexed systems,” IEC 61280-2-9, 2009, describes such an OSNR measurement.
In modern optical communication systems with dense wavelength-division multiplexing (DWDM), various transmitted optical signals are closely spaced in optical frequency, so that it becomes difficult to measure optical noise floor between adjacent signal spectra. This is of a particular concern for communication systems transmitting optical signals at bit rates of 100 Gb/s over 50-GHz wide wavelength channels. In these systems, one needs to measure the optical noise floor within the spectral bandwidth of the signal to determine the signal's OSNR. Such measurements are commonly referred to as in-band OSNR measurements. Furthermore, it is frequently required that these in-band OSNR measurements are performed while the communication system is in service, i.e. that the noise floor within the signal's bandwidth is determined while the optical information signal is transmitted.
Several methods have been disclosed to measure an in-band OSNR in presence of transmitted signals. For conventional single-polarized optical information signals (e.g. for 10 Gb/s NRZ-OOK signals), a polarization nulling technique has been disclosed. Polarization nulling substantially removes the polarized signal from the received noisy signal, thus revealing the floor of an unpolarized optical noise in the spectral bandwidth of the signal. Such a technique has been described in “Optical signal-to-Noise Ratio Measurement in WDM Networks Using Polarization Extinction” by M. Rasztovits-Wiech et al., European Conference on Optical Communication, 20-24 Sep. 1998, Madrid Spain, pp. 549-550.
Modern optical information signals are frequently composed of two mutually orthogonally polarized optical carriers at a same optical frequency. The carriers are independently modulated with digital information data. This polarization multiplexing technique is frequently used in long-distance communication systems to transmit 50 Gb/s BPSK, 100 Gb/s QPSK, or 200 Gb/s 16-QAM signals over 50-GHz wide DWDM channels. In polarization-multiplexed (PM) signals, in-band OSNR cannot be determined by means of the above-referenced polarization-nulling technique, because the two orthogonally polarized optical carriers cannot be simultaneously removed from the noisy optical signal without also extinguishing the optical noise.
While several methods have been disclosed to measure in-band OSNR in polarization-multiplexed signals, they generally only work with optical signals of a predetermined bit-rate, modulation format, and/or signal waveform. Consequently, these methods may be suitable for monitoring of in-band OSNR at certain points in a communication system, e.g. by means of built-in monitoring equipment, but are difficult to use as a general test and measurement procedure. Furthermore, some of these methods are not suitable for determining in-band OSNR in signals substantially distorted by chromatic dispersion (CD) or polarization-mode dispersion (PMD).
By way of example, a method for in-band OSNR measurements, that only works with binary PSK and ASK signals, has been disclosed by X. Liu et al. in “OSNR monitoring method for OOK and DPSK based on optical delay interferometer,” Photon. Technol. Lett., Vol. 19, p. 1172 (2007), as well as by W. Chen et al. in “Optical signal-to-noise ratio monitoring using uncorrelated beat noise,” Photon. Technol. Lett., Vol. 17, p. 2484 (2005), and M. Bakaul in “Low-cost PMD-insensitive and dispersion tolerant in-band OSNR monitor based on uncorrelated beat noise measurement,” Photon. Technol. Lett., Vol. 20, p. 906 (2008). This method does not work with 100 Gb/s PM-QPSK or 200 Gb/s PM-16-QAM signals.
Other methods for in-band OSNR monitoring of polarization-multiplexed signals are based on coherent detection with high-speed receivers and subsequent digital signal processing. These methods typically operate at a predetermined bit-rate. One of these methods, disclosed by T. Saida et al. in “In-band OSNR monitor with high-speed integrated Stokes polarimeter for polarization division multiplexed signal,” Opt. Express, Vol. 20, p. B165 (2012), determines the in-band OSNR from the spread of the four polarization states through which an optical PM QPSK signal cycles rapidly. Clearly, such high-speed polarization analysis requires prior knowledge of the modulation format and the bit-rate of the transmitted signal and, furthermore, is very sensitive to signal distortions caused by chromatic dispersion (CD) and polarization mode dispersion (PMD).
For applications in long-distance communication systems, it may be advantageous to remove CD- and PMD-induced signal distortions prior to determining OSNR. Compensation of signal distortions introduced by CD and PMD may be accomplished electronically in a high-speed digital signal processor 10, which is shown schematically in FIG. 1. In FIG. 1, wavelength-division multiplexed (WDM) signals 11 are coupled to a first polarization beamsplitter (PBS) 12a, and a local oscillator (LO) laser 13 is coupled to a second PBS 12b. The split optical signals are mixed in 90° hybrid mixers 14, converted into electrical signals by photodetectors 15, and digitized by analog-to-digital converters (ADCs) 16. The digital signal processor 10 performs CD compensation 17, PMD compensation 18, and phase recovery 19. Finally, the OSNR is computed at 10a. 
The digital compensation has a disadvantage in that it requires the high-speed ADCs 16, which usually have only a relatively small dynamic range (typically less than 16 dB), thus limiting the OSNR measurement range. In-band OSNR measurement methods employing error vector magnitude (EVM) analysis of the received signal after electronic compensation of CD and PMD have been disclosed by D. J. Ives et al. in “Estimating OSNR of equalised QPSK signals,” ECOC 2011 Technical Digest, paper Tu.6.A.6 (2011) and by R. Schmogrow et al. in “Error vector magnitude as a performance measure for advanced modulation formats,” Photon. Technol. Lett., Vol. 24, p. 61 (2012), as well as by H. Y. Choi et al., “OSNR monitoring technique for PDM-QPSK signals based on asynchronous delay-tap sampling technique,” OFC 2010 Technical Digest, Paper JThA16. EVM analysis intrinsically requires foreknowledge of the particular modulation format of the optical signal.
Another method for OSNR monitoring is based on RF spectral analysis of low-speed intensity variations of polarization-multiplexed signals. This method has been disclosed by H. H. Choi et al. in “A Simple OSNR Monitoring Technique Based on RF Spectrum Analysis for PDM-QPSK Signals,” OECC 2012 Technical Digest (Korea), Paper 6B3-4. However, this method is very sensitive to variations in the signal's waveform. Hence, it requires not only foreknowledge of the modulation format and bit-rate of the analyzed optical signal, but also careful calibration with a noiseless signal.
A method for in-band OSNR measurements using conventional spectral analysis of the optical signal power has been disclosed by D. Gariépy et al. in “Non-intrusive measurement of in-band OSNR of high bit-rate polarization-multiplexed signals,” Opt. Fiber Technol. vol. 17, p. 518 (2011). A disadvantage of this method is that it only works with signals whose optical spectrum is substantially narrower than the spectral width of the DWDM channel, e.g. it works with 40 Gb/s PM NRZ-QPSK signals transmitted through 50-GHz wide DWDM channels, but usually not with 100 Gb/s PM RZ-QPSK signals transmitted through 50-GHz wide DWDM channels.
Yet another method for in-band OSNR measurements in polarization-multiplexed signals has been disclosed by W. Grupp in European Patent EP 2,393,223 “In-band SNR measurement based on spectral correlation,” issued Dec. 7, 2011. This method determines in-band OSNR from measurements of the cyclic autocorrelation function of the signal amplitude, i.e. by calculating noiseless signal power from correlations between spectral components of the Fourier transform of the cyclic autocorrelation function. The cyclic autocorrelation function of the signal's amplitude may be measured, for example, by means of two parallel coherent receivers employing a common pulsed local oscillator laser, as shown schematically in FIG. 2. A pulsed laser 20 is triggered by a clock recovery circuit 21, which receives light via a tap 21a coupled to a 3 dB splitter 22. Electrical signals of the photodetectors 15 are filtered by low-pass filters 23. A variable optical delay 24 is employed to sample the optical signal 11 twice within each symbol period at various times and with various differential delays between the two samples taken within the same symbol period. A processor 25 is used to calculate the OSNR. This method also requires foreknowledge of the modulation format and bit-rate of the optical signal, as well as careful calibration of the apparatus with a noiseless signal. In addition, the method is very sensitive to signal distortions introduced by CD and/or PMD.